Hydrocarbon feedstocks such as aromatic hydrocarbon feedstocks are derived from processes such as naphtha reforming and thermal cracking (pyrolysis). Such feedstocks can be used in a variety of petrochemical processes, such as para-xylene production from an aromatic hydrocarbon feedstock containing benzene, toluene and xylene (BTX), toluene disproportionation, xylene isomerization, alkylation and transalkylation. However, aromatic hydrocarbon feedstocks often contain contaminants comprising bromine-reactive compounds including unsaturated hydrocarbons, such as mono-olefins, multi-olefins and styrenes. These can cause undesirable side reactions in downstream processes. Therefore, these contaminants should be removed from the aromatic hydrocarbon feedstocks before they can be used in other processes.
Improved processes for aromatics production, such as that described in the Handbook of Petroleum Processing, McGraw-Hill, New York 1996, pp. 4.3-4.26, provide increased aromatics yield but also increase the amount of contaminants. For example, the shift from high-pressure semi-regenerative reformers to low-pressure moving bed reformers results in a substantial increase in BI in the reformate streams, which are aromatic hydrocarbon feedstocks for downstream processes. This results in a greater need for more efficient and less expensive methods for removal of hydrocarbon contaminants from aromatic hydrocarbon feedstocks, e.g., reformate streams.
Olefins (mono-olefins and multi-olefins) in aromatic hydrocarbon feedstocks are commercially removed by hydrotreating processes. Commercial hydrotreating catalysts have proved active and stable to substantially convert multi-olefins contained therein to oligomers and to partially convert the olefins to alkylaromatics.
The clay treatment of hydrocarbons is widely practiced in the petroleum and petrochemical industries. Clay catalysts are used to remove impurities from hydrocarbon feedstocks in a wide variety of processes. One of the most common reasons for treating these hydrocarbon feedstocks with a clay catalyst system is to remove undesirable olefins, including both multi-olefins and mono-olefins, in order to meet various quality specifications. As used herein the term “olefinic compound” or “olefinic material” is intended to refer to both mono-olefins and multi-olefins. Olefinic compounds may be objectionable in aromatic hydrocarbons at even very low concentrations of less than a few weight parts per million (wppm) for some processes such as nitration of benzene.
The term “mono-olefins” as used herein means olefinic compounds containing one carbon-carbon double bond per molecule. Examples of mono-olefins are ethylene, propylene, butenes, hexenes, styrene, and octenes. The term “multi-olefins” used herein means olefinic compounds containing at least two carbon-carbon double bonds per molecule. Examples of multi-olefins are butadienes, cyclopentadienes, and isoprenes.
More recently, molecular sieves, and particularly zeolites, have been proposed as replacements for clays in the removal of olefinic compounds from aromatic hydrocarbon feedstocks. U.S. Pat. No. 6,368,496 (Brown et al.) discloses a method for removing bromine reactive hydrocarbon contaminants from aromatic streams by first providing an aromatic feedstream having a negligible diene level. The feedstream is contacted with an acid active catalyst composition under conditions sufficient to remove mono-olefins. An aromatic stream may be pretreated to remove dienes by contacting the stream with clay, hydrogenation or hydrotreating catalyst under conditions sufficient to substantially remove dienes but not mono-olefins.
U.S. Pat. No. 6,500,996 (Brown et al.) discloses a method for the removal of hydrocarbon contaminants, such as dienes and olefins, from an aromatics reformate by contacting an aromatics reformate stream with a hydrotreating catalyst and/or a molecular sieve. The hydrotreating catalyst substantially converts all dienes to oligomers and partially converts olefins to alkylaromatics. The molecular sieve converts the olefins to alkylaromatics. The process provides an olefin depleted product which can be passed through a clay treater to substantially convert the remaining olefins to alkylaromatics. The hydrotreating catalyst has a metal component of nickel, cobalt, chromium, vanadium, molybdenum, tungsten, nickel-molybdenum, cobalt-nickel-molybdenum, nickel-tungsten, cobalt-molybdenum or nickel-tungsten-titanium, with a nickel molybdenum/alumina catalyst being preferred. The molecular sieve is an intermediate pore size zeolite, preferably MCM-22. The clay treatment can be carried out with any clay suitable for treating hydrocarbons.
Aromatic feedstocks having high C8 aromatics (ethylbenzene, para-xylene, meta-xylene, and ortho-xylene), which typically are xylene plant feedstocks for producing para-xylene, ortho-xylene, or mix-xylenes, may be obtained by distillation of reformate streams. Bromine reactive compounds co-boiling with C8 aromatics result in high BI for the C8 aromatic feedstocks. The xylene plant processes and products (e.g., para-xylene) have certain BI requirements or specifications. Conventionally, xylene plant feedstock is treated with an acid treated clay to remove co-boiling olefinic compound(s) under conditions having a temperature range from about 160° C. to about 200° C. and a pressure range from about 1480 to about 2859 kPa-a. Undesired side reactions, e.g., transalkylation or disproportionation, may form benzene as a by-product, which may be a problem for downstream processes, such as separation process by PAREX™. These side reactions are a common problem at the beginning of the process (start-of-run) with the commercial acid treated clay catalysts. After several days on stream, acid treated clay catalysts are highly selective for BI reduction versus transalkylation and disproportionation. The products from commercial xylenes clay treaters typically have benzene levels no more than 30 wppm higher than the benzene in the feed. However, the acid treated clay catalysts have poor stability and catalyst lifetime. As a result, large quantities of acid treated clay are required and must be replaced regularly (typically, every 3 to 12 months for xylene plant feedstocks). Unlike clay catalysts, molecular sieves are known to have high activity for BI reduction, which results in long catalyst lifetime (cycle-length) and high capacity. However, molecular sieves are also known to have high activity for aromatics disproportionation and transalkylation reactions.
For this reason, a need exists for an improved process for reducing BI for xylene plant feedstock that has similar excellent selectivity for BI reduction as clay catalyst but has improved catalyst lifetime. The present invention solves this problem by advantageously contacting the aromatic feedstock with a catalyst comprising a molecular sieve having a zeolite structure type of MWW.